PVC resin could be used in the rigid form, not plasticized, for producing in general pipes, connections, profilates. For increasing flexibility of the polymer, in order to extend application opportunities, it is necessary to blend it with substances named plasticizers; in this manner the polymer can be used to make various products: films, toys, electrical cables, flexible pipes, flooring, etc.
Generally, plasticizers are liquids of high molecular weight, with solvency properties for polymeric products. In order to have a good plasticization, plasticizers must be mixed with the polymer at warm temperature. In this manner it can be absorbed into the amorphous network of the polymer.
In particular, the repeated monomeric unit of PVC is the following:—CH2—CHCl—
The presence of Chlorine, with high electronegativity, into the structure of the monomer gives polarity to the polymer. The long chains —CH2—CHCl are weakly bonded through dipole-dipole bonds between carbon atoms directly bonded to chlorine atoms, with low electronic density, and chlorine atoms of another chain. These secondary bonds, less strong respect to primary covalent bonds, make the product rigid and with poor resistance to flexibility.
Conversely, plasticizers molecules are composed of a polar side, represented normally by carboxylic or aromatic groups, and an apolar side, represented normally by chains of methyl groups —CH2— from C6 to C10. Typical examples are di(2-ethylhexyl) phthalate (DEHP), di-isononyl phthalate (DINP), adipates of 2 ethyl hexyl alcohol phthalate. In any case, hundreds of other substances with plasticizing properties exist, but often they have niche applications because of lower ratio efficiency/cost.
Action of plasticizer like phthalates, referred to PVC is the following: polar carboxylic and aromatics groups interact with polar parts of polymer monomeric unit; apolar parts of plasticizer inserted between polymeric chain increase distance between them. The consequence is that the polymer obtains flexibility properties.
Phthalates, particularly DEHP, have been for long time the more used, because of good mechanical characteristics provided to the manufactures, poor tendency to exudation, even under high temperate and humidity conditions, and low costs compared to all the others. However, the classification of phthalates in the group of dangerous substances (R60-R61 cat. 2) and the insertion into the list of SVHC (substances of very high concern) have induced industry to use phthalates with longer hydrocarbon chain and thus with lower tendency to migrate out the product, as iso nonyl phthalate, but especially, to find new plasticizer with properties of no toxicity and environment safe.
WO 2012/018939 discloses methods for the manufacture of acetals and ketals with high selectivity for acetilization and ketalization over esterification or transesterification reactions. These methods yield the desired product in high yield and of high productivity, free of other side products or contaminants. The method is disclosed for the manufacture of a glycerol ketal of ehyl levulinate. However, the methods herein disclosed are rather complex and need as starting substance the ethyl levulinate, which is a quite complex molecule not particularly suitable as plasticizer.
WO 2010/036884 discloses compounds including polycarbonates, allylic monomers and polymerized or grafted products thereof, oxiranyl functional monomers and polymerized or grafted products thereof, and acrylate and methacrylate monomers and polymerized or grafted products thereof, derived from renewable biomass feedstocks. Said compounds are based on hydroxyl ketal carboxylate precursor, which precursor are known to be used as plasticizers for PCV. However, said compounds and precursors are rather complex to be synthetized and, thus, costly.
JP 10095748 discloses a glycerol-α-polyoxyalkylene glycerol monofatty acid ester to be used as surfactant.
RO 128511 discloses a pour point and limit filtrability temperature depressant additive for gas-oil and to a process for preparing the same. The additive consists of a hydrocarbonated solvent solution containing a copolymer which comprises vinyl acetate dialkylfumarate, the solvent consisting of alkanes, cycloalkanes, mononuclear aromatic or alkyl aromatic hydrocarbons or mixtures thereof, and C—C aliphatic acid ester with (2,2-dimethyl-[1,3]dioxolan-4-yl) methanol solketal.
WO 2009/010527 discloses a process for the preparation of acetals and ketals of glycerol or its monoesters and monoethers in order to reach high yields.
Plasticizers derived from vegetable oils (triglycerides), represent an interesting alternative. However, vegetable oils are not compatible with PVC, because of their low polarity. By treatment of epoxidation of hydrocarbon chains double bonds, oxygen is introduced into the molecule, and this increases compatibility with polar polymers. Therefore, more double bonds are present into the triglyceride, higher is solubility of epoxidized triglyceride.
Also, plasticizing property is increased by length of hydrocarbon chain. Longer is the chain, better is flexibility of product obtained. However, the length of hydrocarbon chain reduces solubility into the polymer network. This is the case of fatty acids, which have chains from C12 to C22: they could impart good flexibility but have poor solubility into PVC resin.
Epoxidation of double bonds of hydrocarbon chain of fatty acids allows to increase solubility into the polymer. The frequency of double bonds is an index of suitability of a fatty acid to be used as plasticizer. For instance, the glyceride of oleic acid, which derives from a C18 fatty acid with one double bond, is less compatible respect to that of linolenic acid with two double bonds.
Soya oil is the more rich of insaturations among the most common vegetable oils. However, it is not possible to introduce more than 15% phr (that means 15 parts for 100 parts of polymer) of epoxidized soya oil into the PVC resin. In fact, over this concentration, oil begins to exude. For this reason, epoxidized oils may be used only as secondary plasticizers (moreover, epoxy function has also stabilizing action on PVC resin).
Epoxidized methyl ester of fatty acids from soya oil, known as biodiesel, has greater compatibility with PVC respect to other epoxidized oils, because it is a monoglyceride and thus has greater capacity to penetrate inside the tangle of polymeric chains. However, oils, which are tri-glycerides, with a molecular weight three times higher than mono-glycerides, have problems to introduce inside polymeric network. In addition, if from one side epoxidized methyl esters have greater solvency properties, on the other side, just by their low molecular weight and thready structure, migrate easier to the outside of polymer, giving it an oily appearance, and causing progressive worsening of product mechanical property (brittling).
In summary, to have a good vegetable origin plasticizer it is necessary to reach an optimum balance among: a) the flexibility of the molecule (property imparted by linear hydrocarbon chains), and b) compatibility with PVC (property imparted by ether bonds or oxirane bonds, while —OH groups are too much polar). Furthermore, the molecule must not migrate outside the polymer matrix (exudation). But good flexibility is in contrast to no exudation: in fact, for instance, epoxidated methyl esters of fatty acids give good flexibility but exude too much.